In modern digital integrated circuits, particularly those fabricated according to the well-known complementary metal- oxide-semiconductor (CMOS) technology, data output circuitry is generally implemented in the form of push-pull drive circuits. As is well known in the art, push-pull output drive circuits include two drive transistors; one drive transistor (the pull-up device) drives the output terminal toward the positive power supply voltage to effect a logic high level, while the second drive transistor (the pull-down device) drives the output terminal toward ground to effect a logic low level. In CMOS circuits, the pull-up device is generally implemented as a p-channel MOS transistor while the pull-down device is implemented as an n-channel MOS transistor. This configuration ensures that no DC current is drawn by the output driver. In addition, use of a p-channel pull-up device allows the output terminal to be driven fully to the power supply voltage, i.e., from "rail-to-rail", as there is no threshold voltage drop across the p-channel pull-up device (as there would be if the pull-up device were an n-channel transistor).
Most MOS integrated circuits fabricated over the last fifteen years have been powered from a nominal 5 volt power supply. However, with the advent of ultra thin gate dielectric layers used in the fabrication of modern MOS transistors, however, many recent integrated circuits are powered from a nominal 3.3 volt power supply. Since both types of circuits remain available and useful in modern digital systems, data must often be communicated from a 5 volt circuit to a 3.3 volt circuit over communication lines or buses. If all integrated circuits in the system were to utilize the same power supply bias, rail-to-rail output levels would be not only acceptable, but preferred. However, if mixed power supply devices are incorporated into the same system, care must be taken that a logic high level signal driven by a 5 volt device does not exceed 3.3 volts, to prevent damage to 3.3 volt devices receiving such a signal.
For the output driver that drives a high output voltage to less than the power supply level, as would be the case for a 5 volt output driver driving a V.sub.OH maximum of 3.3 volts, an n-channel pull-up device may be used, and would be preferable due to the greater mobility of n-channel MOS transistors relative to p-channel MOS transistors. In this case, the gate voltage applied to the n-channel pull-up device (to turn it on) must be above the V.sub.OH minimum level by at least the threshold voltage of the device, while maintaining a reduced V.sub.OH maximum output to the output terminal.
As will be described hereinbelow, it has been discovered that a voltage reference circuit based on a current mirror is useful in providing a stable reference voltage that an output buffer may use in driving the gate of the n-channel pull-up device. Referring now to FIG. 11, a conventional voltage reference generator circuit based on a current mirror is illustrated. The voltage reference circuit of FIG. 11 includes p-channel transistors 1, 3 connected in a current mirror fashion, with their sources biased to V.sub.cc and their gates connected together at the drain of transistor 1. The drain of transistor 1 is connected to the drain of n-channel transistor 7, which has its gate connected to receive a fraction of V.sub.cc determined by voltage divider 5. Transistors 1 and 7 constitute the reference leg of the current mirror. The drain of transistor 3 is connected to the drain and gate of an n-channel transistor 9, at which the reference voltage V.sub.ref is produced. Transistors 3 and 9 thus constitute the mirror leg of the current mirror, as the current conducted by transistor 3 "mirrors" that conducted by transistor 1. The sources of transistors 7, 9 are connected together to current source 11, which conducts a current i.sub.BIAS that, in this example, is the sum of the currents through the reference and mirror legs.
This conventional current mirror-based voltage reference circuit of FIG. 11 is beneficial in certain applications. For example, if transistors 3 and 9 are made to be quite large (i.e., channel width to channel length ratio being relatively large), the output impedance of the circuit of FIG. 11 will be quite low, allowing the circuit to source and sink relatively large currents without significant modulation in the voltage at line V.sub.ref.
However, it is useful to raise the reference voltage by an amount corresponding to the threshold voltage of the n-channel pull-up device in the output driver. The circuit of FIG. 11 does not provide such a threshold voltage shift, as is evident from its construction.
It is therefore an object of the present invention to provide a voltage reference circuit that shifts its output voltage by an n-channel threshold voltage.
It is another object of the present invention to provide such a circuit in which the threshold shift may be done in a way that does not add output impedance to the voltage reference circuit.
It is another object of the present invention to enable the threshold voltage shift to account for body effect of the pull-up device when its body node is biased to a voltage other than its source.
Other objects and advantages of the present invention will be apparent to those of ordinary skill in the art having reference to the following specification together with its drawings.